The invention relates to two-stage, atmospherically supplied burners, for small scale (e.g., residential or light commercial) heating applications, which produce reduced levels of nitrogen oxide (NO.sub.x) emissions. A typical application for such a burner might be in a residential or small commercial water heater.
It is known in the art of fuel combustion that NO.sub.x emission levels may be reduced by staging combustion. Primary, or first stage, combustion is run fuel rich, i.e., there is more fuel gas in the gas/air mixture supplying the first stage than is required for stoichiometric combustion. Additional air is supplied in the second stage to complete combustion.
Because of various factors, however, staged combustion as known in the art is not readily adaptable to the applications to which the current invention is directed. For example, commercial and industrial burners often use powered combustion; residential burners, on the other hand, make use of natural draft, referred to as atmospheric combustion. Atmospheric combustion does not provide the mixing, turbulence, or air/fuel ratio control necessary to effect reduced NO.sub.x levels as currently practiced in the art.
Despite these limitations, atmospherically supplied burners are preferred in the residential or light commercial setting. This is due to their much lower cost, simplicity of design, and ease of maintenance,